I currently have three adult goldies in a 55 gallon tank with two emperor 400 filters. They are about 8", 5-6" and 4". Would it be OK to add another baby? Will the adults hassle he/she? Will I be overstocked?

You can get away with adding another Goldie as you aren't overstocked,and have good filtration.By baby,how small are you talking? You know how Goldies are...If they can fit it into their mouths,they'll eat it!

I've found gold fish to be more peaceful then koi, for example, today I added a small one two a group of 3 bigger ones and 1 middle sized one, all went well, for the first 20min I checked him every 5min and when it came to 30min he was gone, now they couldn_________________Years of fish keeping = Good advice